The present invention relates to method and apparatus for measuring the height position of the melting zone in a blast furnace, and more particularly the invention relates to a method and apparatus designed so that the height level of the upper boundary of a melting zone inside a furnace is measured from the result of an electric measurement of the length of a cable drawn into the furnace from its top and such measurement of the upper boundary height level is accomplished at a plurality of locations continuously over a long period of time during the operation of the furnace so as to more accurately grasp the thermal conditions within the furnace.
During the operation of a blast furnace, in the high temperature area (1000.degree. to 1500.degree. C.) extending from the shaft lower part to the belly and the bosh of the blast furnace is produced a so-called melting zone in which the softening, fusing and dropping of the ores proceed. The recently conducted overhaul investigations of blast furnaces, etc., have shown that since the ore layer and the coke layer exist in the stratified form in the melting zone, the distributed configuration of the layers has an important effect on the distribution of the gas flow in the furnace lower part which is important for the furnace operation and also the height level of the melting zone reflects the thermal conditions of the furnace. For instance, the exemplary furnace conditions reported as the results of the overhaul investigations have shown that in the case of a furnace operated highly efficiently with the fuel ratio of 547 Kg/T.Pig, the melting zone has a sharp inverted V-shape in cross-section (FIG. 1a), and in the case of another blast furnace operated with the low fuel ratio of 470 to 486 Kg/T.Pig the melting zone has a W-shaped cross-sectional shape. Thus it has been proved that there is a close relation between the furnace operation and the position and shape of the melting zone. As a result, to detect and control the level and shape of the melting zone during the operation of a blast furnace is a very important matter for the blast furnace operation since it has the effect of stabilizing the operation and reducing the fuel ratio.
However, since the temperature of the melting zone is high (1000.degree. to 1600.degree. C.), since the inner pressure of recently built large blast furnaces is high (3 to 4 Kg/Cm.sup.2) and since the furnace conditions are severe in that the charge is continuously falling and the melting and fusing of the falling ores, etc., are in the advanced stage, it is extremely difficult to actually measure the position and shape of the melting zone. In view of these circumstances, the conditions within the blast furnaces in operation have been investigated by measuring the temperature, pressure, gas composition, etc., with such means as a vertical sonde, horizontal sonde, etc. However, these measurements have been largely confined to the upper sections of the blast furnaces where the temperature is lower than 1000.degree. C. and no continuous measurements have been made in the high temperature areas near the melting zones. This is due to the fact that since the temperature and pressure are high in the vicinity of the melting zone as mentioned previously, since the measurements by a sonde must be made in the severe conditions where the charge is falling continuously and since the measurement by the sonde involves another difficulties such as the difficulty of maintaining and managing the sonde and the measurements being made intermittently inevitably, up to date the conditions of such melting zones have been merely estimated from the measurements of the low temperature areas lower than 1000.degree. C.
In view of the fact that the importance of sensing the level and shape of a melting zone and controlling the same has been recognized recently, reports have been made of methods using a radioisotope (RI) and methods using a tracer gas only for the purpose of measuring the level and shape of a melting zone. The method of using a solid RI is such that the solid RI is introduced, along with the charge, into a desired location of a furnace from its top and the position of the RI or the position of the melting zone is estimated from the time interval between the time that the RI falling along with the charge is melted in the melting zone and the time that the melted RI is tapped along with the molten metal. However, this method is disadvantageous in that measurements must inevitably be made intermittently in view of the nature of its measuring method and that there is a tendency toward causing an error in the estimation of the time interval between the melting and tapping of the RI. On the other hand, the method of using a tracer gas is designed so that a capsule having a tracer gas sealed therein is introduced into a furnace in the like manner as in the case of the solid RI so that the tracer gas released by the melting of the capsule is moved upward along with the furnace gas and the tracer gas is sensed at the furnace top, thereby estimating the position of the melting from the falling speed of the charge. While this method has the effect of improving the accuracy in estimation of the melting level due to the fact that the time interval between the melting of the capsule and the sensing of the tracer gas can be ignored, the method still appears to be disadvantageous in that the tracer gas is diluted by the large amount of the furnace gas thus giving rise to a difficulty from the standpoint of tracer gas sensing accuracy or a difficulty in terms of the material for the capsule itself, and there has been no examples of the actual use in industrial applications. In addition to these methods, several methods of estimating the level and shape of a melting zone have been proposed in which the estimation is made by means of a model calculation on the basis of the measured values of a pressure gage, temperature indicator, gas analyzer, etc., which are installed in the furnace body. However, there has been no actual case in which the accuracy of the level and shape estimated in this way was proved in comparison with the actual measurements and the measurements were no more than the results of mere estimation. Thus, up to date there has been no method capable of directly and continuously measuring the level and shape of a melting zone in a blast furnace for the purpose of detecting the same. It will thus be seen from the foregoing that despite the great importance of grasping the level and shape of a melting zone, no effective measuring method has been developed as yet. Thus, there has existed a need for a melting zone measuring method so designed that (i) it is capable of continuously measuring the position and shape of a melting zone over a long period of time, and (ii) it is possible to make measurements stably with a high degree of accuracy and it is also possible to directly measure the melting zone.